Telephone systems with reduced acoustic feedback

ABSTRACT

Feedback in an electro-acoustic transducer unit for connection into a two-way communication system is reduced or eliminated by acoustic means dependent on relative positioning of loudspeaker and one or more microphones, or electrically, by cancellation of signals due to unwanted sound components derived from a pair of microphones. The operation of the microphone and loudspeaker can be alternated at an ultrasonic frequency or a single transducer can be used as microphone and loudspeaker alternately at such a frequency. A transducer having a pair of speech coils in pushpull formation is described, as well as a laminated structure for use in the housing of the apparatus.

United States Patent [191 Gabr TELEPHONE SYSTEMS WITH REDUCED ACOUSTICFEEDBACK [75] Inventor: Saad Zaghoul Mohamed Gabr,

London, England [73] Assignee: A.R.D. Anstalt, Vaduz,

Liechtenstein [22] Filed: Dec. 18, 1972 [21] Appl. No.: 315,807

Related US. Application Data [63] Continuation of Ser. No. 1,743, Jan.9, 1970,

abandoned.

[30] Foreign Application Priority Data SOUND SIGNAL SOURCE [1113,889,058 June 10, 1975 2,385,279 9/1945Hopkins..........,................. 179/1 HF 2,691,073 10/1954Lowman......... 179/1 HF 2,736,771 2/1956 Hanson et a1 179/1 HF2,926,221 2/1960 Kagdis 179/1 15.5 DV 3,588,352 6/1971 Yamawaki... 179}!FS 3,601,549 8/1971 Mitchell 179/81 B FOREIGN PATENTS OR APPLICATIONS559,229 3/1957 Italy 179/1 FS Primary ExaminerThomas W. Brown Attorney,Agent, or Firm-Young & Thompson 1 1 ABSTRACT Feedback in anelectro-acoustic transducer unit for connection into a two-waycommunication system is reduced or eliminated by acoustic meansdependent on relative positioning of loudspeaker and one or moremicrophones, or electrically, by cancellation of signals due to unwantedsound components derived from a pair of microphones. The operation ofthe microphone and loudspeaker can be alternated at an ultrasonicfrequency or a single transducer can be used as microphone andloudspeaker alternately at such a frequency. A transducer having a pairof speech coils in push-pull formation is described, as well as alaminated structure for use in the housing of the apparatus.

10 Claims, 14 Drawing Figures 8.888858 PATENTEDJUH I 0 I975 SHEET 3 lREVERSlNG \REVERSING swncn 256 sv c mr azmc 255 SWITCH FIG. 9 265CONTROL SIG HAL SOURCE 34 9 0 2 76/4001- Max/A4150 6140/? TELEPHONESYSTEMS WITH REDUCED ACOUSTIC FEEDBACK This is a continuation ofapplication Ser. No. 1,743, filed Jan. 9, 1970 now abandoned.

The invention relates to the elimination or reduction of feedback inelectro-acoustic transducer apparatus for connection into a two-waycommunication system, for example, a loudspeaking telephone apparatus.

Feedback between the loudspeaker and the microphone is propagated inthree ways, namely by direct radiation, by reflection from the insidesurface of the casing enclosing the loudspeaker and/or microphone and byconduction through the walls of the casing.

It is accordingly an object of the present invention to provide anelectro-acoustic transducer apparatus in which the effects of feedbackare reduced or eliminated.

According to one aspect of the invention, the apparatus comprises afirst transducer means for producing a sound signal in response to anelectrical input, a second transducer means for providing an electricaloutput from sound waves incident thereon, housing means for said firstand second transducer means, and means mounting said first and secondtransducer means in adjacency in said housing means so that theelectrical output from the second transducer means due to sound wavesincident thereon from the first transducer means is zero, said housingmeans and mounting means being adapted to limit sound conduction fromthe first transducer means to the second transducer means.

According to another aspect of the invention, the apparatus comprisesloudspeaker means, two microphone means, housing means, means mountingsaid loudspeaker means and at least one of said microphone means withinsaid housing means, the microphone means being located symmetricallywith respect to the axis of movement of the diaphragm of the loudspeakermeans, and means connecting the outputs of said two microphone means inreverse phase, whereby said connecting means provides an output in whichcomponents due to sounds waves from the loudspeaker means and incidenton the microphone means are cancelled.

The invention also provides an electro-acoustic transducer apparatus,the apparatus comprising a housing means mounting first and secondelectro-acoustic transducer means so that two sides of diaphragm meansof one of the transducer means are substantially equally and directlycoupled to the other transducer means, whereby the electrical outputfrom either transducer means due to the sound waves from the othertransducer means is substantially zero.

The invention will be better understood from the following illustrativedescription and from the accompanying drawings in which:

FIGS. 1, 2 and 3 each schematically show a respective arrangement of aloudspeaker means and microphone means for electro-acoustic transducerapparatus;

FIGS. 4 and 5 are graphs showing the relationship between the absolutepressure applied to the diaphragm of a transducer and the voltage in thecoil thereof.

FIG. 6 schematically shows a transducer having a pair of speech coils inpush-pull formation;

FIG. 7 shows a circuit for a further form of transducer apparatus, inschematic block diagram form;

FIG. 8 is a schematic sectional view of a wall of a housing for atransducer unit; and

FIGS. 9-14 each show in schematic block diagram form a respectivecircuit for an electro-acoustic apparatus.

Referring to FIG. 1, a loudspeaker 210 functions as the reproductionunit and is so positioned with respect to a microphone 212 constitutingthe production unit that the diaphragm of the latter receives the samesound vibrations from the loudspeaker on one side as on the other. Theloudspeaker 210 is fed with an electrical signal corresponding to anunwanted sound, for example, background music from a source 214. Theeffect of the sound vibrations from the loudspeaker 210 on theelectrical output of the microphone 212 is consequently zero. Thelocation of the microphone 212 and the loudspeaker 210 is at the sametime such that the microphone receives, efi'ectively the wanted soundsignal, for example, speech, from a sound source 216 which isconsequently responsible for all or substantially all of the electricaloutput of the microphone appearing across output terminals 215. Thearrangements for example can coveniently be such that the axis of theloudspeaker diaphragm lies on a diameter of the microphone diaphragm,where this is circular, as shown.

It will be appreciated that the microphone and loudspeaker will normallybe housed in a suitable cabinet, which may incorporate associatedequipment to provide all the facilities necessary to a complexcommunication system. In the present instance the axes of theloudspeaker and microphone can both be horizontal, with the latter axisdirected towards the expected position of a sound source to be monitoredor recorded.

Instead of employing acoustic cancellation, electric signal cancellationcan be used by the provision of two microphones. Thus, as shown in FIG.2, two identical microphones 222a and 222b are placed with respect to aloudspeaker 220, so as to produce identical electrical signals from theunwanted sound they receive from it, one microphone 2220 beingcompletely or nearly completely shielded from the wanted sound and theother microphone 222b being exposed to it. In this arrangement also theaxis of the loudspeaker 220 and the common axes of the microphone 222a,222b at right angles thereto are all most conveniently arranged in ahorizontal plane, with the microphone 222b orientated to receive theincoming wanted sound signal from the source 216.

In the alternative arrangement of FIG. 3, which again employs twomicrophones 232a and 232b, the microphone 232a to be shielded from thewanted sound source is placed within a cabinet 238 housing a loudspeaker230 so as to receive the sound produced from the diaphragm. The othermicrophone 232b is positioned externally of the housing in the samerelationship to the loudspeaker 230 as the microphone 212 is to theloudspeaker 210 in FIG. I. The microphone output signals are combinedout of phase as shown so that the electric signal components due to thesound from the loudspeaker 230 are cancelled. The signal component dueto the wanted sound from the source 216 is available terminals 235without or substantially without the addition of a signal due to theloudspeaker reproduction. The circuit arrangements associated with themicrophones can include where appropriate a phase control unit 237, thatis, a unit of a known kind, for relatively adjusting the phases of themicrophone outputs.

The effect of the feedback due to all three of the above mentionedcauses is reduced by shifting the opcrating point of the transducer,whether the micro phone or the loudspeaker, on the power (responsecurve). FIGS. 4 and 5 show the curves for the microphone and loudspeakerrespectively. By shifting the operating point from A to B on each ofthese curves, a greater voltage output to absolute pressure ratio in thecase of the microphone and a greater absolute pressure output to voltageinput ratio are obtained. The actual voice signal is greater than thefeedback signal and, therefore, clue to the shift on the curve, thedifference between the reproduced voice signal and the reproducedfeedback signal is even greater and the effect of the feedback signal islessened.

The shifting of the operating point may be carried out as shown in FIG.6 by fitting the transducer 50 with identical coils 52 on either side ofthe diaphragm 54 and applying a DC biassing voltage to the coils as froma source 55. The coils 52 may be the actual speech coils or alternatelyseparate biassing coils may be provided. The use of the two speech coilswith the transducer increases the output by at least twice; by usingspecial magnets, a further increase is possible. The fact that theoutput from the transducer is doubled means that the overall gain of theamplifier systems can be reduced and therefore the stability of thecircuit would be increased.

A further reduction of feedback is produced by operating thearrangements of FIGS. 1, 2 and 3 so that the microphone and loudspeakerare active alternately by providing a suitable high-speed switchingsystem. The effect of this is that a closed circuit between microphoneand loudspeaker is never formed as the loudspeaker never operates at thesame time as the microphone. Also, spurious signal reflection effectsfrom the transducer enclosure are reduced.

Alternatively, as illustrated in FIG. 7, operation of a microphone 252for receiving the wanted sound interrupted at a high frequency and aloudspeaker 250 operated only when the microphone is inoperative. Thefre' quency at which the operation of the two transducers is alternatedor interruptions are made is variable and may, for example, by 15,000 or16,000 per second.

This aspect of the invention can be conveniently carried into effect byelectronic, for example transistorized, switch means 254 operated at therequired frequency under control of a frequency source 256. The switchmeans 254 acts to cut off the loudspeaker 250 from the electric signalsource 214, at the same time connecting the microphone 252 to theassociated output terminal 215, and then to connect the loudspeaker andsource at the same time connecting the micro phone to the terminals,this cycle being repeated. In an alternative arrangement of this generalkind, a receiver amplifier and a separate microphone amplifier areenergised from a single source alternatively as by an alternating squarewave, so that only one of the amplifiers is working at any instant.

Due to the mechanical inertia of the microphone and loudspeaker, thisalternate activation is not apparent to the user of the station.However, provided a suitable rate of alternation is chosen, echos fromthe surround ings of the transducers used are reduced.

The feedback due to conduction through the casing is reduced by the typeof easing used. A preferred type of easing construction is shown in FIG.8 and consists ofa rigid layer 600, an air gap 601 and a soft layer 602.Further rigid and soft layers separated by an air gap may be used, thegreater the number of layers, the greater the reduction of conductedfeedback.

In the further embodiments of the invention illustrated in FIGS. 9 and10, the functions of the first and second transducer means are performedby a single transducer means alternately. The transducer means, which asbefore may comprise one or more actual transducer units, is thusarranged to operate as a microphone for an instant, and as a loudspeakerduring a following instant. The frequency at which these functionsalternate is again advantageously at least l0,000 cycles per second andpreferably greater than 15,000 c.p.s.

Both FIGS. 8 and 9 schematically illustrate complete communicationsystems. The system of FIG. 8 has one transducer means 260, 261 at eachof a pair of stations. Each station is further provided with a reversingswitch 262, 264 which can be incorporated in the body of the associatedtransducer operable to connect this transducer with either of a pair oflines 265, 266 including amplifiers 267, 268 respectively. The switches262, 264 are arranged to alternate connection of both transducers 260with the line 265, when the transducer 260 acts as a microphone and thetransducer 261 acts as a loudspeaker with connection of both transducersto the line 266, when the functions of the two transducers areinterchanged. The positions of the switches 262, 264 must of course beco-ordinated, as by means of a coordinating or synchronising signalsupplied to the switches 262, 264 from a suitable source 269. Alternatively, the two switches can be operated independently at very differentfrequencies, for example l0,000 and 30,000 c.p.s., generated randomly,so that the connections required are obtained during a sufficient partof the time for communication to be effected.

In the alternative arrangement shown in FIG. 10 only a single amplifieris used. The two stations of the system illustrated include respectiveelectro-acoustic transducer means 270, 271 connected to a switch means272. The switch means is arranged to alternate connections of thetransducer 270, 271 to the input terminal 274 and the output terminals275 of an amplifier 276. In one position of the switch means, thetransducer 270 is connected to the input terminal 274 and functions as amicrophone whilst the other transducer 271 is connected to the outputterminal 275 and functions as a loudspeaker. In the other position theconnections and functions of the two transducers are interchanged.

It will be understood that the switch means 272 although represented asa two pole, two way mechanical switch is preferably constituted as anelectronic switch, such as a transistorised switch, as are the switchmeans 262, 264 of FIG. 9.

The arrangements illustrated in FIGS. 11-14 each comprise a singleinstrument or station for incorporation in a conventional single linetelephone system. In FIG. 11, the instrument comprises a singletransducer means 280, a switch means 282 and amplifiers 284, 285connected by a hybrid junction 286 for transmission into the singleoutput line 287. This arrangement permits signal controlling thealternating function of the transducer means 280 to be fed from a source288 into the line 287 at any convenient point on its length. The need toconnect the synchronizing signal source 269 of FIG. 9 to both switchmeans 262, 264 by a separate line is thus eliminated.

In FIG. 12, a transducer apparatus 290 is provided which can compriseany one of the arrangements described above with reference to FIGS. 1-3.The two transducer means of the apparatus 290 are connected torespective amplifiers 291, 292. The amplifier outputs are taken to ahybrid junction circuit 294. It will be evident that in the apparatus ofFIG. 12, both transducers function continuously, the elimination fromthe electrical output of a component due to unwanted sound beingeffected by electrical or acoustic cancella tron.

FIG. 13 shows apparatus in which the output of a transducer apparatus300, the apparatus comprising an arrangement in accordance with FIGS.1-3, is fed to a pair of amplifiers 301, 302. Alternate energisation ofthe amplifiers 301, 302 is effected by a control unit 304.Alternatively, a control unit can be supplied which blocks the inputsalternately. The arrangement illustrated includes a hybrid circuit 305for feeding into the line. A modification of this arrangement is shownin FIG. 14 in which alternate operation of the transducer units of thetransducer arrangement 310, which again comprises an apparatus asdescribed in connection with FIGS. 1-3, is effected by switching circuit312 which can take any of the forms described in connection with theearlier Figures. A microphone amplifier 314 and a loudspeaker amplifier316 which are both permanently energised in use are again connected atthe output ends to a hybrid unit 317.

It will be understood that although reference has been made to use ofsingle loudspeakers and microphones in the arrangements described above,a plurality of such units can be employed in place of each if desired.The apparatus is not limited to loudspeakers and microphones of anyparticular kind.

The invention can thus be variously embodied to provide an electricoutput signal representing exclusively or nearly exclusively the wantedcomponent of an incoming sound containing also unwanted components. Thesignal can be recorded for subsequent replay or fed directly to aloudspeaker when appropriate. The electrical output signal can bemonitored to enable the amplitude of the cancellation signal to beadjusted to the required level.

I claim:

1. In combination, housing means, a single loudspeaker having adiaphragm having an axis of movement and an electrical input, microphonemeans having diaphragm means with exposed opposite sides and anelectrical output, means mounting said single loudspeaker and saidmicrophone means in close proximity in said housing means so that soundwaves radiated by the loudspeaker diaphragm in response to energizationof the loudspeaker travel in a direction parallel to the sides of themicrophone diaphragm means directly and by a single sound path to themicrophone means, the microphone diaphragm means being disposedsubstantially in a plane lying along the axis of movement of theloudspeaker diaphragm when the loudspeaker diaphragm moves in responseto an electrical input to the loudspeaker, said opposite sides of saidmicrophone diaphragm means being equidistant from said loudspeakerdiaphragm, whereby the sound waves radiated by the loudspeaker diaphragmfall directly and substantially equally on said opposite sides of themicrophone diaphragm means, so that the electrical output of themicrophone means due to the sound waves from the single loudspeakerdiaphragm is substantially zero.

2. The combination claimed in claim 1, in which said microphone meanscomprises a pair of microphones back to back and having oppositelyoutwardly facing diaphragms.

3. The combination claimed in claim 2, said microphone diaphragms beingcoaxial.

4. The combination claimed in claim 1, said microphone means lying onsaid axis of movement of said loudspeaker diaphragm.

5. The combination claimed in claim 1 including a second microphonehaving a diaphragm and means mounting said second microphone in saidhousing means in close proximity to said loudspeaker on the side of theloudspeaker diaphragm remote from said first-mentioned microphone withthe diaphragm of said second microphone disposed in a plane lying alongsaid axis of movement of said loudspeaker diaphragm.

6. The combination claimed in claim 5 having circuit means connectingsaid first-mentioned and said second microphone in reverse phase toprovide said common electrical output.

7. The combination claimed in claim 6 in which said circuit meansincludes a phase adjustment means.

8. The combination claimed in claim 1 in which said housing meansincludes at least one wall comprising laminations of hard and relativelysoft material.

9. The combination claimed in claim 1 including electric circuit meansinterconnecting said microphone and said loudspeaker and adapted tooperate said microphone and said loudspeaker alternately at anultrasonic frequency.

10. The combination claimed in claim 1 including a second microphone,means mounting said second microphone in said housing means in closeproximity to said loudspeaker, circuit means connecting together saidfirst-mentioned microphone and said second microphone and providingtherefor a common electrical output, and a phase adjustment meansincluded in said circuit means.

1. In combination, housing means, a single loudspeaker having adiaphragm having an axis of movement and an electrical input, microphonemeans having diaphragm means with exposed opposite sides and anelectrical output, means mounting said single loudspeaker and saidmicrophone means in close proximity in said housing means so that soundwaves radiated by the loudspeaker diaphragm in response to energizationof the loudspeaker travel in a direction parallel to the sides of themicrophone diaphragm means directly and by a single sound path to themicrophone means, the microphone diaphragm means being disposedsubstantially in a plane lying along the axis of movement of theloudspeaker diaphragm when the loudspeaker diaphragm moves in responseto an electrical input to the loudspeaker, said opposite sides of saidmicrophone diaphragm means being equidistant from said loudspeakerdiaphragm, whereby the sound waves radiated by the loudspeaker diaphragmfall directly and substantially equally on said opposite sides of themicrophone diaphragm means, so that the electrical output of themicrophone means due to the sound waves from the single loudspeakerdiaphragm is substantially zero.
 2. The combination claimed in claim 1,in which said microphone means comprises a pair of microphones back toback and having oppositely outwardly facing diaphragms.
 3. Thecombination claimed in claim 2, said microphone diaphragms beingcoaxial.
 4. The combination claimed in claim 1, said microphone meanslying on said axis of movement of said loudspeaker diaphragm.
 5. Thecombination claimed in claim 1 including a second microphone having adiaphragm and means mounting said second microphone in said housingmeans in close proximity to said loudspeaker on the side of theloudspeaker diaphragm remote from said first-mentioned microphone withthe diaphragm of said second microphone disposed in a plane lying alongsaid axis of movement of said loudspeaker diaphragm.
 6. The combinationclaimed in claim 5 having circuit means connecting said first-mentionedand said second microphone in reverse phase to provide said commonelectrical output.
 7. The combination claimed in claim 6 in which saidcircuit means includes a phase adjustment means.
 8. The combinationclaimed in claim 1 in which said housing means includes at least onewall comprising laminations of hard and relatively soft material.
 9. Thecombination claimed in claim 1 including electric circuit meansinterconnecting said microphone and said loudspeaker and adapted tooperate said microphone and said loudspeaker alternately at anultrasonic frequency.
 10. The combination claimed in claim 1 including asecond microphone, means mounting said second microphone in said housingmeans in close proximity to said loudspeaker, circuit means connectingtogether said first-mentioned microphone and said second microphone andproviding therefor a common electrical output, and a phase adjustmentmeans included in said circuit means.